One of the recognized problems of excimer lasers is contamination of the lasing gas composition. contamination can occur from impurities created during laser operation and by the various contaminants introduced into the lasing gas composition from the components used in the laser, such as, for example, plastics. To deal with these sorts of impurities, a number of elaborate purification methods and devices have been developed including those described in U.S. Pat. Nos. 4,964,137, 5,073,896 and 5,090,020, the complete text of which are hereby incorporated by reference. Amongst the undesirable impurities that these methods and devices were developed to remove include CO.sub.2, Cl.sub.2, CO, CCl.sub.4, SiF.sub.4, HF, NO, NO.sub.2, N.sub.2 O.sub.4, H.sub.2 O, NH.sub.3, N.sub.2, O.sub.2, H.sub.2, CF.sub.2 O, OF.sub.2, CH.sub.3 Cl, CH.sub.4, and CH.sub.3 F.
Another source of potential contamination, not specifically addressed by the aforementioned patents, is imparted during the manufacture of the gas. Contaminants may be introduced during gas handling and mixing.
Gas manufacturers have always attempted to minimize the content of impurities in their gas. While certain maximum tolerances are often established by manufacturers for impurities such as N.sub.2, O.sub.2, HF and H.sub.2, manufacturers traditionally attempt to keep the amount of such impurities well below maximum tolerance. For example, the inventor has always attempted to maintain O.sub.2 below 5 ppm, although published tolerance was 25 ppm. Manufacturers have also attempted to maintain consistently low levels of contaminants such as oxygen well below any published tolerances from batch to batch to help foster predictable and repeatable laser performance.
But, despite their best efforts to maintain strict quality control specifications, inexplicably, some cylinders of lasing gas composition produce acceptable performance, i.e. consistent and high levels of output and lasing efficiency, and some do not. One aspect of the present invention is the identification of a cause of that variability.
The inventor undertook an extensive analysis of rare gas halide lasing compositions which provided unstable lasing performance in an attempt to determine the source of the instability. After analyzing the data, the present inventor was struck by a trend in the characteristics exhibited by excimer lasers using "acceptable" and "unacceptable" batches of lasing composition. It was apparent to the inventor that the first quantity of lasing composition introduced into the excimer laser from cylinders containing otherwise "unacceptable" lasing compositions always produced the best performance.
To resolve this minor mystery, one cylinder of rare gas halide lasing composition which had provided unstable lasing performance was subjected by the inventor to a test in which the cylinder was repeatedly unhooked and re-attached to the laser each time a lasing run was finished. Normal air purging procedures were followed each time the tank was re-attached to remove the air introduced into the gas feed assembly. Despite purging, however, the inventor discovered that this procedure had the effect of letting some air into the laser, air which was not completely expelled with the subsequent introduction of additional lasing composition. Amazingly, the output power of the laser did not decrease between each run as previously observed. In short, the laser exhibited a more stable power output. The inventor therefore concluded that some component of air was a required additive to stabilize excimer laser performance.
Additional tests were conducted whereby standard lasing compositions were doped with HF, hydrogen, synthetic air, natural air (wet) and oxygen. It was determined that synthetic air, natural air (wet) and oxygen made the laser work better, i.e. provided more stable performance. Nitrogen and HF had no noticeable impact on the laser performance. This test established that not all of the components of naturally occurring air provided additional stabilized performance to excimer lasers.
To confirm that oxygen was responsible for the increased stability and performance of an excimer laser, two new batches of lasing composition were produced containing three and five ppm of oxygen respectively. When these formulations were introduced into an excimer laser, the results were poor. Ten ppm of oxygen was then added to one cylinder and 15 ppm of oxygen was added to the other. The resulting lasing compositions were re-introduced into an excimer laser and the excimer laser exhibited exemplary performance. The inventor has therefore discovered that oxygen, or oxygen containing compounds, formally thought to be contaminants to be minimized, are actually necessary for providing stable lasing performance to rare gas halogen excimer lasers. The inventor does not currently know why the introduction of O.sub.2 or other oxygen containing compounds stabilizes an excimer laser's output. The inventor does know, however, that the introduction of O.sub.2 as described herein does not in any way change the nature of the excimer, i.e. the laser still emits at the same frequency.